As is known, starting with an AC current source, a current-source power converter can be used to obtain, by virtue of an active rectifier module, a constant DC current on a power supply bus having an inductor. In a variable speed drive application, the constant current flowing in the power supply bus is then chopped by an inverter module so as to obtain a variable output current intended to supply an electrical load.
A current-source power converter comprising a rectifier module connected upstream to several inputs supplied by a current source, a power supply bus connected downstream of the rectifier and provided with a positive line and with a negative line, an inductor being connected to the positive line and/or to the negative line of the power supply bus, and an inverter module connected downstream of the power supply bus and designed to supply a variable current to an electrical load is known from the publication entitled “Design and Performance of a 200 kHz All-SiC JFET Current Source Converter” by Thomas Friedli, Simon D. Round, Dominik Hassler and Johann W. Kolar. The rectifier module comprises several switching legs connected between the positive line and the negative line of the power supply bus, each switching leg comprising two transistors in series and a connection middle-point located between the two transistors and linked to one of the inputs. The rectifier module also comprises several switching legs connected between the positive line and the negative line of the power supply bus, each switching leg comprising two transistors in series and a connection middle-point located between the two transistors and linked to the electrical load to be controlled. In the aforementioned publication, the transistors of the rectifier module and of the inverter module are normally-on SiC JFETs, that is to say that they conduct when no voltage is applied to their gate. Normally-on components are very suitable for use in current-source power converters because this type of converter requires two transistors to be permanently conducting, so that the current source is never connected to an open circuit.
However, when using normally-on components it is necessary to prevent the mains from short-circuiting during start-up or during malfunction of the auxiliary power supply, the latter being intended to control the gates of the transistors.
In the aforementioned publication, to prevent the mains from short-circuiting it was suggested to place, in the input stages connected to the mains, upstream of the rectifier, relays switched to the open state during start-up. However, this solution is unsatisfactory because the three relays present in the input stages must be sized to withstand all of the current from and all of the mains voltage.